How disease can wipe out an entire species

Rat study presents first evidence for extinction due to ‘hyperdisease’

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This illustration shows the rat species Rattus nativitatis, which went extinct on Australia's Christmas Island by 1908. In a new study of museum DNA samples, researchers report that the likely cause of the animals' extinction was an introduced disease.

Disease can wipe out an entire species, reveals a new study on rats native to Australia's Christmas Island that fell prey to "hyperdisease conditions" caused by a pathogen that led to the rodents' extinction.

The study, published in the latest issue of the journal PLoS One, presents the first evidence for extinction of an animal entirely because of disease.

The researchers say it's possible for any animal species, including humans, to die out in a similar fashion, although a complete eradication of Homo sapiens would be unlikely.

"I can certainly imagine local population or even citywide 'extinction,' or population crashes due to introduced pathogens under a condition where you have a pathogen that can spread like the flu and has the pathogenicity of the 1918 flu or Ebola viruses," co-author Alex Greenwood, assistant professor of biological sciences at Old Dominion University in Norfolk, Va., told Discovery News.

The 1918 flu killed millions of people, while Ebola outbreaks have helped to push gorillas close to extinction.

For the Christmas Island study, Greenwood and his colleagues collected DNA samples from the island's now-extinct native rats, Rattus macleari and R. nativitatis, from museum-housed remains dating to both before and after the extinction event, which occurred between 1899 and 1908.

Co-author Ross MacPhee, a curator of vertebrate zoology at the American Museum of Natural History in New York, N.Y., explained that Charles Andrews of the British Museum documented at the time that black rats were first brought to the island via the S.S. Hindustan in 1899. The ship-jumping black rats then carried a protozoan known as Trypanosoma lewisi. A related organism causes sleeping sickness in humans.

"Fleas are the intermediate host for one of the developmental stages of Trypanosoma, and the only likely method (of disease spread) is infected fleas crossing from black rats to endemic rats," MacPhee told Discovery News.

After the Hindustan's arrival, the native island rats were observed staggering around deathly ill on footpaths. Shortly thereafter, they were never seen again.

The museum DNA samples showed that Christmas Island native rodents collected before the black rats invaded the island were not infected with the protozoan, but six out of 18 collected post-contact were infected.

Eight great extinct species"Not every rat would have to be infected," Greenwood explained. "If you push a population down to an unsustainable number then it will collapse. In addition, if a substantial number of reproducing individuals became infected and ill, even if they survived the infection, their reproduction rate may be lowered and lead to a population crash."

Given the rats' fate, scientists are concerned about Tasmanian devils, which have been dying in record numbers due to devil facial tumor disease, a contagious cancer for which the carnivorous marsupials appear to have no immunity.

Such island species seem to be more vulnerable to extinction by disease. In a prior study, MacPhee determined that at least 80 percent of all species-level losses during the past 500 years have occurred on islands.

"The general explanation for islander susceptibility would presumably be that island denizens live in a sort of bubble, protected by water barriers from diseases prevalent on mainlands or elsewhere," MacPhee explained. "But when the bubble is broken -- think measles epidemics in Iceland in the 19th century -- the mortality can be extreme."

Karen Lips, associate professor of zoology at Southern Illinois University, told Discovery News that the new research was "well done and convincing, despite the limited number of samples available."

She also pointed out that island-like conditions exist within mainland areas.

"I work up on mountaintops, another kind of island with high endemism, which is greatly affected by emerging infectious disease," she said.

Elk in North America, for example, have suffered worrisome population losses due to wasting diseases induced by prions. Various South Pacific fruit bats and amphibians are also under threat now due to infectious diseases.

"What can be done?" asked MacPhee.

"Probably nothing other than captive conservation," he added. "Most wildlife biologists are hoping that such diseases, although severe, will eventually accommodate and the species will pull through."